1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high powered pulse lasers, in shock processing of solid materials, and more particularly to methods and apparatus for changing the vibrational frequency and mode shapes of workpieces, such as gas turbine engine blades to improve blade lifetime, and the engine operating envelope.
2. Description of the Related Art
Airfoil failures, i.e., cracking, separation, and distortion, result from a number of different causes, one being vibratory resonance. Gas turbine engine blades occasionally respond in their operating envelope by resonant mode vibration between their start, idle and maximum RPM speeds.
As shown in FIG. 1, there are many possible, ideal fundamental vibration mode shapes that a turbine blade may undergo. Particular sources of low order excitation include inlet distortion, bleed ducts, and frame or vane struts. Such resonance modes may be plotted in relationship between blade frequency and rotor speed as shown in FIG. 2.
Elastic or aerodynamic coupling of blades within a compressor stage, may result in mode vibrations. Furthermore, other types of aerodynamic instability, such as flutter or the like, may create an unstable or self-feeding resonance within the blade.
At particular resonance conditions, these gas turbine engine blades may be subjected to very high amplitude beating, therefore, creating high fatigue stresses at the resonance nodes. In operation, such gas turbine engines are constructed to be operated away from such resonant frequencies of the blades. This, however, is difficult when confronted with many resonance modes in the blade.
Engine designers sometimes find that where they wish to operate a gas turbine engine (for greatest efficiency), a problem arises in that at such speed, resonance conditions occur for at least one set of engine blades.
What is needed in the art is the ability to shift or alter the resonance frequency of a gas turbine engine blade.